This invention relates to an organotin catalyst system for promoting the reaction of organic isocyanates, i.e. NCO-containing compounds, with compounds having groups containing reactive hydrogen, as determined by the Zerewitinoff method described in J. Am. Chem. Soc., Vol. 49, p. 3181 (1927). The organotin catalyst system has particular utility in the preparation of polyurethanes, a broad class of organic polymers formed by the reaction of di- or poly- isocyanates with a number of difunctional or polyfunctional compounds having hydroxyl groups containing active hydrogen, e.g. water, polyols, polyethers, polyesters, and the like.
One effective class of catalysts to promote such reactions within reasonable reaction times have been the reaction products of carboxylic acids and dialkyl tin oxides, such as, for example, the dialkyl tin carboxylates disclosed in U.S. Pat. No. 3,392,128, and the Harada complex disclosed in U.S. Pat. Nos. 3,661,887 and 3,676,402.
It has now been ascertained that inclusion of a bis(tri-n-alkyl)oxide with the aforementioned catalysts provides a synergistic effect such that the catalyst concentration can be reduced without effecting the cure rate. Furthermore, this catalyst system appears to provide polyurethanes having improved hydrolytic and thermal stability over those prepared utilizing the carboxylic acid/dialkyl tin oxide catalyst alone.